Planographic printing is a printing process employing property that oil and water repel each other. There are non-image portions receiving water but repelling oily ink and image portions receiving oily ink but repelling water on the surface of the planographic printing plate.
An aluminum support used in the planographic printing plate material is required to have conflicting properties of high hydrophilicity and high water retention on one hand and high adhesion to an image formation layer provided thereon on the other hand. A support with poor hydrophilicity attracts ink at non-image portions, resulting in so-called background contamination. A support with poor water retention, when the supply amount of a dampening solution to a printing plate is small, produces background contamination, resulting in lowering of water tolerance.
Recently, CTP, in which a digital image data is directly written in a planographic printing plate material through a laser has been developed and put into practical use. As a laser light source is used a visible light source having a longer wavelength such as an Ar laser (488 nm), an FD-YAG laser (532 nm) or a high power semiconductor laser having an emission wavelength not less than 750 nm. Further, a semiconductor laser (hereinafter also referred to as a violet laser) employing, for example, InGaN or ZnSe type material, which can continuously emit light with a wavelength of from 380 to 430 nm, is about to be put into practical use.
A planographic printing plate material with an image formation layer meeting the respective laser has been also developed.
An aluminum support for a planographic printing plate is generally manufactured by surface roughening an aluminum plate surface according to a process comprising an appropriate combination of steps selected from mechanical surface roughening, chemical etching in an acidic or basic solution, desmutting in an acidic solution, electrolytical surface roughening, anodizing in an acidic solution, hydrophilizing treatment, and sealing treatment.
Particularly, the electrolytical surface roughening has been generally used as a surface roughening method of an aluminum support for a planographic printing plate, since a uniformly roughened surface is easily obtained. The electrolytical surface roughening is ordinarily carried out in an aqueous hydrochloric acid solution or in an aqueous nitric acid solution.
Various manufacturing processes of the aluminum support for a planographic printing plate have been proposed (see, for example, Japanese Patent O.P.I. Publication Nos. Japanese Patent O.P.I. Publication Nos. 2003-039846, 2003-019877, 2003-019878, 2003-362046, and 2003-363799, and U.S. Pat. Nos. 5,122,243 and 5,186,795).
However, a planographic printing plate material, comprising an aluminum support manufactured according to these processes, and an image formation layer provided thereon, is insufficient in adhesion of the image formation layer to the support, and a planographic printing plate, which is prepared from such a planographic printing plate material according to a plate making process comprising imagewise exposure by laser, has problems in that small dot reproduction or printing durability is poor. Particularly when printing is carried out employing ink containing no VOC, the planographic printing plate provides poor printing durability or poor water tolerance.